Inkjet printing

ABSTRACT

The present disclosure relates to an Inkjet printing process that comprises inkjet printing an inkjet ink composition onto a substrate to form a printed inkjet ink layer. The inkjet ink composition comprises a colorant, a curable polyurethane dispersion, a photoinitiator and water, wherein the amount of curable polyurethane dispersed in the inkjet ink composition is 0.1 to 30 weight %. A radiation-curable overcoat composition is applied over the printed inkjet ink layer as an overcoat layer. The overcoat composition comprises a curable polyurethane dispersion, a photoinitiator and water. The printed inkjet ink layer is then cured on the substrate by exposing both the overcoat layer and inkjet ink layer on the substrate to radiation.

BACKGROUND

Inkjet printing is a printing method that utilizes electronic signals tocontrol and direct droplets or a stream of ink onto print media. Inkjetprinting may involve forcing ink drops through small nozzles by thermalejection, piezoelectric pressure or oscillation onto the surface of themedia. This technology can be used to record images on various mediasurfaces (e.g. paper).

In inkjet printing, curable polymer binders may be added to inkjet inksto improve the durability of the resulting print. Such binders may becured, for example, by exposure to radiation e.g. UV radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

Various implementations are described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating, by way of example, anexample of a printing process of the present disclosure.

DETAILED DESCRIPTION

Before the present disclosure is disclosed and described, it is to beunderstood that this disclosure is not limited to the particular processsteps and materials disclosed in this disclosure because such processsteps and materials may vary. It is also to be understood that theterminology used in this disclosure is used for the purpose ofdescribing particular examples. The terms are not intended to belimiting because the scope is intended to be limited by the appendedclaims and equivalents thereof.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in this disclosure, “overcoat” in the context of the presentdisclosure refers to a composition that is applied to a print substrateas an under layer to the inkjet ink composition. The overcoatcomposition may be substantially colourless, clear or transparent.Accordingly, the overcoat composition may be substantially free frompigment or other colorants, and may have no substantive effect on thecolour of an underlying coloured image printed from inkjet ink.

As used in this disclosure, “acidity,” “acid number,” or “acid value”refers to the mass of potassium hydroxide (KOH) in milligrams thatneutralizes one gram of a substance. The acidity of a polymer can bemeasured according to standard techniques, for example as described inASTM D1386. If the acidity of a particular polymer is specified, unlessotherwise stated, it is the acidity for that polymer alone, in theabsence of any of the other components of the overcoat or inkjet inkcomposition.

As used herein, the term “transparent” is used to describe a compositionthat allows light to pass therethrough. In some examples, in the contextof an overcoat composition, the term “transparent” may mean that thecomposition allows light to pass through it such that, when the overcoatcomposition is applied over a printed image of at a thickness of 3microns or less, for instance, 1.5 to 2 microns (e.g. 1.5 microns), theprinted image may be clearly visible to the naked eye. In some examples,the overcoat composition may be transparent, whereby, when the overcoatcomposition is printed over a printed image of at a thickness of 1.5microns, the colours in the image are substantially the same as thecolours in the uncoated image. In some examples, the difference in thecolour(s) of the coated and un-coated image are small. Reference is madeto ASTM D1729-96 (Reapproved 2009, which specifies the equipment andprocedures for visual appraisal of colours and colour differences ofopaque materials that are diffusely illuminated. In some examples, thedelta E (determined according to CIE94) between the colours of thecoated and un-coated image may be 3 or less, for example, 2 or less. Insome examples, the delta E (determined according to CIE94) may be 1.5 orless, for example, 1 or less.

Optical density or absorbance is a quantitative measure expressed as alogarithmic ratio between the radiation falling upon a material and theradiation transmitted through a material.

${A_{\lambda} = {- {\log_{10}\left( \frac{I_{1}}{I_{0}} \right)}}},$

where A_(λ) is the absorbance at a certain wavelength of light (λ), I₁is the intensity of the radiation (light) that has passed through thematerial (transmitted radiation), and I₀ is the intensity of theradiation before it passes through the material (incident radiation).The incident radiation may be any suitable white light, for example, daylight or artificial white light. The optical density or delta E of animage may be determined using methods that are well-known in the art.For example, optical density and/or delta E may be determined using aspectrophotometer. Suitable spectrophotometers are available under thetrademark X-rite.

As used in this disclosure, the term “about” is used to provideflexibility to a numerical range endpoint by providing that a givenvalue may be a little above or a little below the endpoint to allow forvariation in test methods or apparatus. The degree of flexibility ofthis term can be dictated by the particular variable and would be withinthe knowledge of those skilled in the art to determine based onexperience and the associated description in this disclosure.

As used in this disclosure, a plurality of items, structural elements,compositional elements, and/or materials may be presented in a commonlist for convenience. However, these lists should be construed as thougheach member of the list is individually identified as a separate andunique member. Thus, no individual member of such list should beconstrued as a de facto equivalent of any other member of the same listsolely based on their presentation in a common group without indicationsto the contrary.

Concentrations, amounts, and other numerical data may be expressed orpresented in this disclosure in a range format. It is to be understoodthat such a range format is used merely for convenience and brevity andthus should be interpreted flexibly to include not just the numericalvalues explicitly recited as the limits of the range, but also toinclude all the individual numerical values or sub-ranges encompassedwithin that range as if each numerical value and sub-range is explicitlyrecited. As an illustration, a numerical range of “about 1 wt % to about5 wt %” should be interpreted to include not just the explicitly recitedvalues of about 1 wt % to about 5 wt %, but also include individualvalues and subranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3.5, and 4 andsub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This sameprinciple applies to ranges reciting a single numerical value.Furthermore, such an interpretation should apply regardless of thebreadth of the range or the characteristics being described.

The present disclosure relates to an inkjet printing process thatcomprises inkjet printing an inkjet ink composition onto a substrate toform a printed inkjet ink layer. The inkjet ink composition comprises acolorant, a curable polyurethane dispersion, a photoinitiator and water,wherein the amount of curable polyurethane dispersed in the inkjet inkcomposition is 0.1 to 30 weight %. A radiation-curable overcoatcomposition is applied over the printed inkjet ink layer as an overcoatlayer. The overcoat composition comprises a curable polyurethanedispersion, a photoinitiator and water. The printed inkjet ink layer isthen cured on the substrate by exposing both the overcoat layer andinkjet ink layer on the substrate to radiation.

The present disclosure also relates to a printed substrate comprising anink layer comprising a colorant disposed over the substrate and anovercoat layer disposed over the ink layer, wherein the printedsubstrate comprises a crosslinked polyurethane network that surroundsthe colorant and extends from the ink layer to the overcoat layer.

Polyurethane dispersions may be used as curable polymer binders inaqueous UV-curable inkjet ink compositions. However, it can be difficultto achieve adequate durability without compromising othercharacteristics of the inkjet ink composition. For example, while highlevels of polymer binder can improve durability, excessive levels ofpolymer binder can affect the jettability of an inkjet ink composition.The latter can have a negative effect on the printed image, as well ason the lifespan of the printhead. Moreover, aqueous UV-curable inkscomprising e.g. yellow or black colorants may be difficult to curebecause these colorants may be strong absorbers of UV light. Whilecuring may be improved by increasing the polyurethane and/orphotoinitiator content of the inkjet ink, this may have a detrimentaleffect on the jettability of the ink composition.

In the present disclosure, a polyurethane dispersion is included in anovercoat composition that is applied as an overcoat layer over theinkjet ink composition. This can allow higher levels of polyurethane tobe deposited onto the substrate to enhance durability. At the same time,the polyurethane content of the inkjet ink composition can be kept belowa threshold to maintain desired levels of jettability. Since theovercoat composition need not be applied digitally using a printhead,high levels of polyurethane can be employed without the risk ofcompromising jettability. The relative concentrations of polyurethane inthe overcoat composition and inkjet ink composition may be tailored toprovide the desired levels of durability, while maintaining jettabilityof the inkjet ink composition. The presence of an overcoat layer overthe inkjet ink layer may also reduce or eliminate exposure of the inkjetink layer to oxygen, facilitating cure of the inkjet ink layer.

Furthermore, in the present disclosure, both the overcoat compositionand the inkjet ink composition comprise curable polyurethane. Thus, whenthe overcoat and the inkjet ink are exposed to radiation,photoinitiators in the overcoat and inkjet ink generate reactive speciese.g. radicals. These reactive species react to cure the polyurethane inthe overcoat and inkjet ink, forming a crosslinked polyurethane networkthat extends from the inkjet ink layer to the overcoat layer. Thiscrosslinked polyurethane network can help to retain colorant on thesubstrate, improving the durability of the printed image on thesubstrate. In some examples, reactive groups of the polyurethane in theovercoat layer crosslink with reactive groups of the polyurethane in theink layer. In some examples, the crosslinked polyurethane network maysurround the colorant in the ink layer.

Overcoat

The overcoat composition comprises water, a photoinitiator and a curablepolyurethane dispersion. The overcoat composition may additionallycomprise a surfactant. The overcoat composition may be an analogueovercoat composition.

Water may be present in the overcoat composition in an amount of atleast 30 weight %, for example, at least 40 or 50 weight %. In someexamples, water may be present in the overcoat composition in an amountof at least 60 weight %. Water may be present in an amount of at most 99weight %, for example, at most 95 weight %. In some examples, water maybe present in the overcoat composition in an amount of 30 to 99 weight%, for instance, 40 to 98 weight % or 50 to 95 weight %. In otherexamples, water may be present in an amount of 60 to 93 weight %, forinstance, 70 to 90 weight %.

Any suitable photoinitiator may be employed in the overcoat composition.The photoinitiator initiates the polymerization and/or crosslinking ofthe radiation-curable polyurethane upon exposure to radiation. Thephotoinitiator may be the same or different from the photoinitiatoremployed in the inkjet ink composition. Suitable photoinitiators aredescribed in relation to the overcoat composition below. However, forthe avoidance of doubt, the water-soluble photoinitiators described inrelation to the inkjet ink composition may be used in the overcoatcomposition, if desired.

Some examples of the photoinitiator include1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (whichis commercially available from BASF Corp. as IRGACURE® 2959); acylphosphine oxide photoinitiators (e.g., IRGACURE® 819, commerciallyavailable from BASF Corp.); alpha hydroxy ketone photoinitiators (e.g.,IRGACURE® 184, commercially available from BASF Corp.); Iodonium,(4-methylphenyl)[4-(2-methylpropyl) phenyl]-, hexafluorophosphate(I-)(which is commercially available from BASF Corp. as IRGACURE® 250); ahigh-molecular-weight sulfonium salt (e.g., IRGACURE® 270, commerciallyavailable from BASF Corp.);2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (which iscommercially available from BASF Corp. as IRGACURE® 369); alpha aminoketone photoinitiator (e.g., IRGACURE® 379, commercially available fromBASF Corp.); a liquid blend of alpha hydroxy ketone/benzophenonephotoinitiator (e.g., IRGACURE® 500, commercially available from BASFCorp.); and a liquid photoinitiator blend of acyl phosphine oxide/alphahydroxy ketone (e.g., IRGACURE® 2022, commercially available from BASFCorp.). Some other suitable photoinitiators include phosphine oxidederivatives, thioxanthone derivatives, and benzophenone derivatives.

The photoinitiator may be present in the overcoat composition in anamount ranging from about 0.1 wt % to about 20 wt. % of the total wt %of the overcoat composition. In another example, the photoinitiator ispresent in the in the inkjet ink in an amount ranging from about 0.2 wt% to about 15 wt %, for example, 0.5 wt % to 10 wt % or 0.5 to 5 wt % ofthe total wt % of the overcoat composition.

Any suitable curable polyurethane dispersion may be included in theovercoat composition. The polyurethane dispersion may be UV-curable, forexample, curable by UV-LED. Suitable UV-LED wavelengths include 365 nm,385 nm, 395 nm or 405 nm. In one example, the polyurethane dispersionmay be curable by UV-LED at 365 nm, 385 nm or 395 nm. In one example,the polyurethane dispersion may be curable by UV-LED at 395 nm.

The polyurethane may be the same as or different from the polyurethanepresent in the inkjet ink composition. Suitable polyurethanes aredescribed in relation to the overcoat composition below. However, forthe avoidance of doubt, the pH stable polyurethane dispersions describedbelow in relation to the inkjet ink composition may be used to form theovercoat composition, if desired.

In some examples, polyurethane dispersions comprise polyurethane polymerparticles dispersed in water. The particles may range from about 20 toabout 200 nm in size. The polyurethane can have a molecular weight (Mw)in the range of about 1,000 to 100,000 or in the range of about 5,000 toabout 50,000. The polyurethane may have a NCO/OH ratio of 1.2 to 5 andan acid number of 20 to 100. The double bond density of the polyurethanemay be 1.5 to 20, for example, 2 to 10.

The polyurethane may be formed from the reaction of a diisocyanate and apolyol. The diisocyanate can be an aliphatic diisocyanate or an aromaticdiisocyanate. Examples of suitable diisocyanates include methylenediphenyl diisocyanate, hexamethylene diisocyanate, p-tetramethyl xylenediisocyanate, m-tetramethyl xylene diisocyanate, bitolylenediisocyanate, toluene diisocyanate,methylene-bis(4-cyclohexyl)diisocyanate, p-phenylene diisocyanate,isophorone diisocyanate, 1,5-naphthalene diisocyanate and mixturesthereof.

In some examples, the polyol can be a diol selected from the group of:cyclic diols (e.g. 1,3-cyclohexanedimethanol and1,4-cyclohexanedimethanol); aliphatic polycarbonate diols; polyetherdiols; polyethylene glycol; polypropylene glycol; polytetramethyleneglycol; poly(ethylene oxide) polymers; poly(propylene oxide) polymers;poly(tetramethylene oxide) polymers; copolymers thereof having terminalhydroxyl groups derived from polyhydric compounds including diols; andcombinations thereof. In one aspect, the diol can be a cyclic diol. Inanother aspect, the diol can be an aliphatic cyclic diol.

Additionally or as an alternative to the polyols mentioned above, thepolyol may an acrylate-containing diol or a methacrylate-containingdiol. Examples include:

In some examples, the polyurethane is a water-dispersible acrylicfunctional polyurethane. In some other examples, polyurethane is awater-dispersible (meth)acrylated polyurethane. Suitablewater-dispersible (meth)acrylated polyurethane are commerciallyavailable under the trademarks Ucecoat®6558, Ucecoat®6559, Ebecryl®2002and Ebecryl®2003 (Cytec).

In some examples, the polyurethane dispersions are water-dispersible(meth)acrylated polyurethane, sold under the trade name of NeoRad® R441by NeoResins (Avecia). Other representative but non limiting examples ofsuitable polyurethane dispersions include Ucecoat®7710, Ucecoat®7655(available from Cytec), Neorad®R440, Neorad®R441, NeoradeR447,Neorad®R448 (available from DSM NeoResins), Bayhydrol®UV 2317,Bayhydrol®UV VP LS 2348 (available from Bayer), Lux430, Lux®399, Lux®484(available from Alberdingk Boley), Laromer®LR8949, Laromer®LR8983,Larome® PE22WN, Laromer®PE55WN, Laromer®UA9060 (available from BASF).

The amount of polyurethane (solids) dispersed in the overcoatcomposition may be 0.1 to 50 weight % or 0.5 to 40 weight %, forexample, 1 to 30 weight %. In some examples, the amount of polyurethane(solids) in the overcoat composition may be 2 to 25 weight %, forinstance, 5 to 15 weight %. As the overcoat composition may not beapplied by inkjet printing, the polyurethane content may be relativelyhigh as jettability may not be a concern. The polyurethane content(solids) of the overcoat composition may be greater than thepolyurethane content (solids) of the ink composition.

The overcoat composition may be formed using a pre-formed orcommercially available polyurethane dispersion comprising polyurethanedispersed in its own solvent (e.g. water). The amount of polyurethanedispersion used to form the overcoat composition may be 5 to 30 weight %of the total weight of the overcoat composition.

Any suitable surfactant may be present in the overcoat composition.Where the inkjet ink composition also includes a surfactant, thesurfactant present in the overcoat may be the same or different from thesurfactant in the inkjet ink composition.

Suitable surfactants may include non-ionic, cationic, and/or anionicsurfactants. Examples include a silicone-free alkoxylated alcoholsurfactant such as, for example, TEGO® Wet 510 (Evonik Tego Chemie GmbH)and/or a self-emulsifiable wetting agent based on acetylenic diolchemistry, such as, for example, SURFYNOL® SE-F (Air Products andChemicals, Inc.). Other suitable commercially available surfactantsinclude SURFYNOL® 465 (ethoxylated acetylenic diol), SURFYNOL® CT 211(non-ionic, alkylphenylethoxylate and solvent free), and SURFYNOL® 104(non-ionic wetting agent based on acetylenic diol chemistry), (all ofwhich are from Air Products and Chemicals, Inc.); ZONYL® FSO (a.k.a.CAPSTONE®, which is a water-soluble, ethoxylated non-ionicfluorosurfactant from Dupont); TERGITOL™ TMN-3 and TERGITOL™ TMN-6 (bothof which are branched secondary alcohol ethoxylate, non-ionicsurfactants), and TERGITOL™ 15-S-3, TERGITOL™ 15-S-5, and TERGITOL™15-S-7 (each of which is a secondary alcohol ethoxylate, non-ionicsurfactant) (all of the TERGITOL™ surfactants are available from The DowChemical Co.). Fluorosurfactants may also be employed.

When present, the surfactant present in the overcoat composition in anamount ranging from about 0.01 wt % to about 5 wt % based on the totalwt % of the overcoat composition.

The overcoat composition may contain less than 0.01 wt % colorant, forexample, less than 0.001 wt % colorant. The overcoat composition may besubstantially free from colorant. In some examples, the overcoatcomposition may be substantially free from pigment or dye.

The overcoat composition may be transparent.

The overcoat composition may be made in-situ just before being appliedto the substrate.

Inkjet Ink Composition

The inkjet ink composition comprises water, a colorant, a photoinitiatorand a curable polyurethane dispersion. The inkjet ink composition mayadditionally comprise a surfactant.

Water may be present in the inkjet ink composition in an amount of atleast 30 weight %, for example, at least 40 or 50 weight %. In someexamples, water may be present in the inkjet ink composition in anamount of at least 60 weight %. Water may be present in an amount of atmost 99 weight %, for example, at most 95 weight %. In some examples,water may be present in the inkjet ink composition in an amount of 30 to99 weight %, for instance, 40 to 98 weight % or 50 to 95 weight %. Inother examples, water may be present in an amount of 60 to 93 weight %,for instance, 70 to 90 weight %.

Any suitable colorant may be used in the inkjet ink composition. Thecolorant may be a pigment or a dye. In some examples, the colorant canbe present in an amount from about 0.5 wt % to about 15 wt % based on atotal wt % of the inkjet ink composition. In one example, the colorantcan be present in an amount from about 1 wt % to about 10 wt %. Inanother example, the colorant can be present in an amount from about 5wt % to about 10 wt %.

In other examples, the colorant may be a pigment or dye. In someexamples, the colorant may be a pigment. As used herein, “pigment”generally includes organic or inorganic pigment colorants, magneticparticles, aluminas, silicas, and/or other ceramics, organo-metallics orother opaque particles, whether or not such particulates impart color.Thus, although the present description primarily illustrates the use ofpigment colorants, the term “pigment” can be used more generally todescribe pigment colorants, as well as other pigments such asorganometallics, ferrites, ceramics, etc.

Suitable pigments include the following, which are available from BASFCorp.: PALIOGEN® Orange, HELIOGEN® Blue L 6901F, HELIOGEN® Blue NBD7010, HELIOGEN® Blue K 7090, HELIOGEN® Blue L 7101F, PALIOGEN® Blue L6470, HELIOGEN® Green K 8683, HELIOGEN® Green L 9140, CHROMOPHTAL®Yellow 3G, CHROMOPHTAL® Yellow GR, CHROMOPHTAL® Yellow 8G, IGRAZIN®Yellow 5GT, and IGRALITE® Rubine 4BL. The following pigments areavailable from Degussa Corp.: Color Black FW1, Color Black FW2, ColorBlack FW2V, Color Black 18, Color Black, FW200, Color Black 5150, ColorBlack S160, and Color Black 5170. The following black pigments areavailable from Cabot Corp.: REGAL® 400R, REGAL® 330R, REGAL® 660R,MOGUL® L, BLACK PEARLS® L, MONARCHS 1400, MONARCHS 1300, MONARCH 1100,MONARCH 1000, MONARCH® 900, MONARCH 880, MONARCH 800, and MONARCH 700.The following pigments are available from Orion Engineered Carbons GMBH:PRINTEX® U, PRINTEX® V, PRINTEX® 140U, PRINTEX® 140V, PRINTEX® 35, ColorBlack FW 200, Color Black FW2, Color Black FW 2V, Color Black FW 1.Color Black FW 18, Color Black S 160, Color Black S 170, Special Black6, Special Black 5, Special Black 4A. and Special Black 4. The followingpigment is available from DuPont: TI-PURE® R-101. The following pigmentsare available from Heubach: MONASTRAL® Magenta, MONASTRAL® Scarlet,MONASTRAL® Violet R, MONASTRAL® Red B, and MONASTRAL® Violet Maroon B.The following pigments are available from Clariant: DALAMAR® YellowYT-858-D, Permanent Yellow GR, Permanent Yellow G, Permanent Yellow DHG,Permanent Yellow NCG-71, Permanent Yellow GG, Hansa Yellow RA, HansaBrilliant Yellow 5GX-02, Hansa Yellow-X, NOVOPERM® Yellow HR, NOVOPERM®Yellow FGL, Hansa Brilliant Yellow 10GX, Permanent Yellow G3R-01,HOSTAPERM® Yellow H4G, HOSTAPERM® Yellow H3G, HOSTAPERM® Orange GR,HOSTAPERM Scarlet GO, and Permanent Rubine F6B. The following pigmentsare available from Sun Chemical: QUINDO® Magenta, INDOFAST® BrilliantScarlet, QUINDO® Red R6700, QUINDO® Red R6713, INDOFAST® Violet,L74-1357 Yellow, L75-1331 Yellow, L75-2577 Yellow, and LHD9303 Black.The following pigments are available from Birla Carbon: RAVEN® 7000,RAVEN® 5750, RAVEN® 5250, RAVEN® 5000 Ultra® II, RAVEN® 2000, RAVENS1500, RAVENS 1250, RAVENS 1200, RAVENS 1190 Ultra®. RAVENS 1170, RAVENS1255, RAVENS 1080, and RAVENS 1060. The following pigments are availablefrom Mitsubishi Chemical Corp.: No. 25, No. 33, No. 40, No. 47, No. 52,No. 900, No. 2300, MCF-88, MA600, MA7, MA8, and MA100. The colorant maybe a white pigment, such as titanium dioxide, or other inorganicpigments such as zinc oxide and iron oxide.

Specific examples of a cyan colour pigment may include C.I. Pigment Blue−1, −2, −3, −15, −15:1, −15:2, −15:3, −15:4, −16, −22, and −60.

Specific examples of a magenta colour pigment may include C.I. PigmentRed −5, −7, −12, −48, −48:1, −57, −112, −122, −123, −146, −168, −177,−184, −202, and C.I. Pigment Violet-19.

Specific examples of a yellow pigment may include C.I. Pigment Yellow−1, −2, −3, −12, −13, −14, −16, −17, −73, −74, −75, −83, −93, −95, −97,−98, −114, −128, −129, −138, −151, −154, and −180. While severalexamples have been given herein, it is to be understood that any otherpigment or dye can be used that is useful in modifying the colour of theUV curable ink.

Specific examples of black pigment include carbon black pigments. Anexample of an organic black pigment includes aniline black, such as C.I.Pigment Black 1.

In some examples, the pigment may be a cyan, magenta, black or yellowpigment.

It has been found that certain colorants e.g. black colorants can bestrong absorbers of radiation (e.g. UV radiation). As a result they canbe more difficult to cure. The amount of polyurethane in the inkjet inkcomposition and/or the overcoat composition may be varied depending onthe nature of the colorant, for example, to ensure adequate curing andprint durability.

Any suitable photoinitiator may be employed. The photoinitiator may bepresent in the inkjet ink composition in an amount ranging from about0.1 wt % to about 10 wt % based on a total wt % of the inkjet inkcomposition.

The photoinitiator may be the same or different from the photoinitiatoremployed in the inkjet ink composition. Suitable photoinitiators aredescribed in relation to the overcoat composition above. However, insome examples, a water-soluble photoinitiator described in relation tothe inkjet ink composition may be used.

The water soluble photoinitiator may be atrimethylbenzoylphenylphosphinic acid metal salt (i.e., TPA salt) havinga formula (I) of

where n is any integer from 1 to 5 and M is a metal with a valence from1 to 5. Examples of suitable metals include Li, Na, K, Cs, Rb, Be, Mg,Ca, Ba, Al, Ge, Sn, Pb, As, and Sb.

The TPA salt may be formed from ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPO-L) and a metal salt. The ethyl(2,4,6-trimethylbenzoyl) phenylphosphinate may be added to a suitablesolvent (e.g., methyl ethyl ketone (MEK)) to form a solution, and thenthe metal salt may be added to the solution. The solution may be heatedand stirred at a predetermined temperature for a predetermined time toallow the reaction to take place. As a result of the reaction, a solidTPA salt may form. This salt may be collected, washed, and dried.

Two example synthetic pathways for forming a lithium TPA salt (TPA-Li)and a sodium TPA salt (TPA-Na) are shown in the schemes below:

The solubility of the water soluble photoinitiator disclosed herein maybe high. In one example, the water soluble photoinitiator can have awater solubility of at least 0.1 wt %. When the water solubility is atleast 0.1 wt %, it means that of the total wt % of the water solublephotoinitiator added to w-ter, at least 0.1 wt % of the total is watersoluble. In some instances, the water soluble phoinitiator may have awater solubility of at least 0.5 wt %. In some instances, the watersoluble photoinitiator may have a water solubility up to about 20 wt %.

The water soluble photoinitiator may be used in combination with asensitizer. The sensitizer may be a water soluble polymeric sensitizerthat includes a functionalized anthrone moiety, a polyether chain, andan amide linkage or an ether linkage attaching one end of the polyetherchain to the functionalized anthrone moiety. The anthrone moiety may bea thioxanthrenone moiety.

In one example, the polymeric sensitizer may have a formula (Q):

where R₁, R₂, R₃, R₄, and R₅ are each independently selected from thegroup consisting of a hydrogen atom, a substituted or unsubstitutedalkyl group, a substituted or unsubstituted allyl group, a substitutedor unsubstituted alkene or alkenyl group, a substituted or unsubstitutedaryl group, a substituted or unsubstituted aralkyl group, a halogenatom, —NO₂, —O—R_(d), —CO—R_(d), —CO—O—R_(d), —O—CO—R_(d),—CO—NR_(d)R_(e), —NR_(d)R_(r), —NR_(d)—CO—R_(e), —NR_(d)—CO—O—R_(e),—NR_(d)—CO—NR_(e)R_(f), —SR_(d), —SO₂—R_(d), —SO₂—R_(d), —SO₂—O—R_(d),—SO₂NR_(d)R_(e) and a perfluoroalkyl group. R_(d), R_(r), and R_(f) areeach independently selected from the group consisting of a hydrogenatom, a substituted or unsubstituted alkyl group, a substituted orunsubstituted allyl group, a substituted or unsubstituted alkene oralkenyl group, a substituted or unsubstituted aryl group, and asubstituted or unsubstituted aralkyl group. Some examples of suitablealkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl,pentyl, hexyl, etc. One example of a suitable alkene group is anethylene group. Some examples of suitable aryl groups include phenyl,phenylmethyl, etc. In the formula Q above, X is O, S, or NH and thepolyether chain has n number of repeating monomer units, where n rangesfrom 1 to 200. As shown in the formula Q above, the linkage is an etherlinkage.

When present, the sensitizer may be present in an amount of 0.1 wt % toabout 10 wt % of the inkjet ink composition.

Any suitable curable polyurethane dispersion may be used to form theinkjet ink composition. For example, a pre-formed curable polyurethanedispersion may be added to the remaining ink components. Such apre-formed curable polyurethane dispersion may comprise polyurethanedispersed in its own solvent (e.g. water). The curable polyurethanedispersion may be present in the inkjet ink composition in an amount of0.5 to 20 weight % or 2 to 15 weight %, for example, 3 to 12 weight %.

The curable polyurethane (solids) that is dispersed in an inkjet inkcomposition may be present in the inkjet ink composition an amount of0.1 to 30 or 20 weight % or 0.1 to 10 weight %, for example, 0.5 to 7weight %, or 0.6 to 5 weight % of the total weight of the inkjet inkcomposition.

Suitable curable polyurethanes are described in relation to the overcoatcomposition above. The curable polyurethane in the inkjet inkcomposition may be the same or different from the curable polyurethanein the overcoat composition. In some examples, the curable polyurethaneis a pH stable polyurethane. pH stable polyurethane may form pH stablepolyurethane dispersions that may be resistant to hydrolysis. In someexamples, the pH of the pH stable polyurethane dispersions remainssubstantially stable even when the dispersions are stored for aprolonged period of time. Thus, inkjet inks formed using such pH stablepolyurethanes may have a longer shelf-life, as the tendency for pigmentsdispersed in the inkjet ink composition to crash out of the inkjet inkcomposition may be reduced.

The pH stable polyurethane may comprise a polyurethane polymercomprising a polyurethane backbone having at least one terminal (orcapping) group selected an acrylamide-containing group, astyrene-containing group, an allyl-containing group:

For avoidance of doubt the CAPS and CHES terminal groups above may be inanionic form, namely:

Where the terminal group is selected from CAPS or CHES, these cappinggroups (CAPS) and (CHES) may be formed by reacting a polyurethanepre-polymer with 3-(cyclohexylamino)-1-propanesulfonic acid and/or2-(cyclohexylamino)ethanesulfonic acid. The3-(cyclohexylamino)-1-propanesulfonic acid and2-(cyclohexylamino)ethanesulfonic acid may react with terminal —N═C═Ogroups on the polyurethane pre-polymer. These capping groups may help tostabilise the polyurethane dispersion.

Where the terminal or capping group is an acrylamide,acrylamide-containing capping group may be CH₂═CHC(O)NH(CH₂)_(n)O—,wherein n is an integer from 1 to 10. In some examples, n is 1 to 6, forinstance, 1 to 4. In one example, the acrylamide-containing cappinggroup may be a group of the formula below:

The acrylamide-containing group may be formed by reacting a polyurethanepre-polymer with an acrylamide-containing monoalcohol or monoamine. Forexample, the acrylamide-containing monoalcohol may react with terminal—N═C═O groups on the polyurethane pre-polymer. An example of a suitableacrylamide-containing mono-alcohol may be:

Where the capping group comprises a styrene-containing group, suitablestyrene-containing capping groups may include:

The styrene-containing groups (e.g. Groups (II) to (VI) above) may beformed by reacting suitable styrene-containing mono-alcohols ormono-amines with a polyurethane pre-polymer. For example, thestyrene-containing monoalcohol may react with terminal —N═C═O groups onthe polyurethane pre-polymer. The corresponding styrene-containingmono-alcohols for Groups (II) to (VI) above may be:

Where the capping group comprises an allyl-containing group, theallyl-containing group may comprise an allyl ether group or an allylamine group. Suitable allyl-containing capping groups include:

The allyl-containing groups (e.g. Groups (VII) to (XIII) above) may beformed by reacting suitable allyl-containing mono-alcohols or monoamineswith a polyurethane pre-polymer. For example, the allyl-containingmonoalcohol or monoamine may react with terminal —N═C═O groups on thepolyurethane pre-polymer. The corresponding allyl-containingmono-alcohols or monoamines may be:

In one example, the polyurethane dispersion may comprise polyurethanepolymers that comprise polyurethane backbones having terminal groupsselected from acrylamide-containing groups, styrene-containing groups,and allyl-containing groups. The dispersion may be devoid ofpolyurethane polymers that comprise polyurethane backbones havingterminal methacrylate-containing or acrylate-containing groups.

In one example, the polyurethane dispersion may comprise at least onepolyurethane polymer comprising a polyurethane backbone having at leastone terminal (or capping) group selected from an acrylamide-containinggroup, a styrene-containing group, an acrylate-containing group, amethacrylic-containing group and an allyl-containing group, and at leastone polyurethane polymer comprising a polyurethane backbone having atleast one terminal (or capping) ionic group; and/or at least onepolyurethane polymer comprising a polyurethane backbone that is cappedat one end with a terminal (or capping) group selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group and at the opposite end with a terminal ionicgroup.

In the polyurethane that is dispersed in the ink and/or overcoatcomposition, 1 to 99 weight % of the capping groups may be ionic groups,while 99 to 1 weight % of the capping groups may be selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group. In some examples, 5 to 70 weight % of thecapping groups may be ionic groups, while 95 to 30 weight % of thecapping groups may be selected from an acrylamide-containing group, astyrene-containing group, an acrylate-containing group, amethacrylic-containing group and an allyl-containing group. In someexamples, 10 to 50 weight % of the capping groups may be ionic groups,while 90 to 50 weight % of the capping groups may be selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group. In other examples, 20 to 40 weight % of thecapping groups may be ionic groups, while 80 to 60 weight % of thecapping groups may be selected from an acrylamide-containing group, astyrene-containing group, an acrylate-containing group, amethacrylic-containing group and an allyl-containing group.

In one example, the polyurethane dispersion may comprise at least onepolyurethane polymer comprising a polyurethane backbone that is cappedat one end with a terminal (or capping) group selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group and capped at the opposite end with a terminalionic group.

In one example, the polyurethane dispersion may comprise (i) at leastone polyurethane polymer comprising a polyurethane backbone that iscapped at one end with a terminal (or capping) group selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group and capped at the opposite end with a terminalionic group; and (ii) at least one polyurethane polymer comprising apolyurethane backbone that is capped at both ends with a terminal (orcapping) group selected from an acrylamide-containing group, astyrene-containing group, an acrylate-containing group, amethacrylic-containing group and an allyl-containing group or (iii) atleast one polyurethane polymer comprising at least one polyurethanepolymer comprising a polyurethane backbone that is capped at both endswith ionic groups.

In one example, the polyurethane dispersion may comprise (i) at leastone polyurethane polymer comprising a polyurethane backbone that iscapped at one end with a terminal (or capping) group selected from anacrylamide-containing group, a styrene-containing group, anacrylate-containing group, a methacrylic-containing group and anallyl-containing group and capped at the opposite end with a terminalionic group; (ii) at least one polyurethane polymer comprising apolyurethane backbone that is capped at both ends with a terminal (orcapping) group selected from an acrylamide-containing group, astyrene-containing group, an acrylate-containing group, amethacrylic-containing group and an allyl-containing group and (iii) atleast one polyurethane polymer comprising at least one polyurethanepolymer comprising a polyurethane backbone that is capped at both endswith ionic groups.

The ionic group may comprise a carboxylic acid group, a carboxylategroup, a sulphonic acid group and/or a sulphonic acid group. The ionicgroup may be formed by reacting an amino carboxylic acid or an aminosulphonic with a polyurethane pre-polymer, for example, with terminal—N═C═O groups on the polyurethane pre-polymer. Suitable amino sulphonicacids include taurine, CAPS or CHES (see above). The ionic groups mayhelp to keep the polyurethane particles in dispersion in water.

In one example, the polyurethane dispersion may comprise at least onepolyurethane polymer comprising a polyurethane backbone having at leastone terminal (or capping) group selected from a methacrylic-containinggroup and/or an acrylate-containing group, and at least one polyurethanepolymer comprising a polyurethane backbone having at least one terminal(or capping) ionic group selected from

and/or at least one polyurethane polymer comprising a polyurethanebackbone that is capped at one end with a terminal (or capping) groupselected from a methacrylic-containing group and/or anacrylate-containing group, and at the opposite end with a terminal groupselected from CAPS or CHES above. In the polyurethane that is dispersedin the ink and/or overcoat composition 1 to 99 weight % of the cappinggroups may be CAPS and/or CHES, while 99 to 1 weight % of the cappinggroups may be selected from an acrylate-containing group and/or amethacrylic-containing group. In some examples, 5 to 70 weight % of thecapping groups may be CAPS and/or CHES, while 95 to 30 weight % of thecapping groups may be selected from an acrylate-containing group and/ora methacrylic-containing group. In some examples, 10 to 50 weight % ofthe capping groups may be CAPS and/or CHES, while 90 to 50 weight % ofthe capping groups may be selected from an acrylate-containing groupand/or a methacrylic-containing group. In some examples, 20 to 40 weight% of the capping groups may be CAPS and/or CHES, while 80 to 60 weight %of the capping groups may be selected from an acrylate-containing groupand/or a methacrylic-containing group.

In one example, the polyurethane dispersion may comprise at least onepolyurethane polymer comprising a polyurethane backbone that is cappedat one end with a terminal (or capping) group selected from anacrylate-containing group and/or a methacrylio-containing group andcapped at the opposite end with a terminal CAPS or CHES group.

In one example, the polyurethane dispersion may comprise (i) at leastone polyurethane polymer comprising a polyurethane backbone that iscapped at one end with a terminal (or capping) group selected from anacrylate-containing group and/or a methacrylio-containing group, andcapped at the opposite end with a terminal CAPS or CHES group; and (ii)at least one polyurethane polymer comprising a polyurethane backbonethat is capped at both ends with a terminal (or capping) group selectedfrom an an acrylate-containing group and/or a methacrylic-containinggroup or (iii) at least one polyurethane polymer comprising at least onepolyurethane polymer comprising a polyurethane backbone that is cappedat both ends with terminal groups selected from CAPS and/or CHES groups.

In one example, the polyurethane dispersion may comprise (i) at leastone polyurethane polymer comprising a polyurethane backbone that iscapped at one end with a terminal (or capping) group selected from anacrylate-containing group and/or a methacrylio-containing group andcapped at the opposite end with a terminal CAPS or CHES group; (ii) atleast one polyurethane polymer comprising a polyurethane backbone thatis capped at both ends with a terminal (or capping) group selected froman an acrylate-containing group and/or a methacrylio-containing groupand (iii) at least one polyurethane polymer comprising at least onepolyurethane polymer comprising a polyurethane backbone that is cappedat both ends with terminal groups selected from CAPS and/or CHES groups.

Suitable acrylate- or methacrylate-containing capping groups may include

Groups (XIII) to (XIVI) above may be formed by reacting thecorresponding methacrylate/acrylate-containing mono-alcohols with apolyurethane pre-polymer, for example, with —N═C═O terminal groups onthe pre-polymer.

The polyurethane backbone of the polyurethane polymers present in the pHstable polyurethane dispersion may be formed from the reaction between areactive diol and a diisocyanate. The reactive diol may be selected froman acrylate-containing diol, a methacrylate-containing diol,acrylamide-containing diol, styrene-containing diol and/or anallyl-containing diol.

Suitable methacrylate-containing and acrylate-containing reactive diolsinclude:

The reactive diol may also be a styrene-containing reactive diolselected from:

The reactive diol may also be an allyl-containing containing diolselected from:

Suitable diisocyanates include methylene diphenyl diisocyanate,hexamethylene diisocyanate, p-tetramethyl xylene diisocyanate,m-tetramethyl xylene diisocyanate, bitolylene diisocyanate, toluenediisocyanate, 4,4′-Methylene dicyclohexyl diisocyanate, p-phenylenediisocyanate, isophorone diisocyanate, 1,5-naphthalene diisocyanate,trimethylhexamethylene diisocyanate and mixtures thereof.

In one example, the diisocyanate is selected from at least one of 2,2,4-trimethyl hexamethylene diisocyanate, isophorone diisocyanate,hexamethylene diisocyanate, methylene diphenyl diisocyanate and4,4′-Methylene dicyclohexyl diisocyanate.

In one example, a blend of 4,4′-Methylene dicyclohexyl diisocyanate andhexamethylene diisocyanate is used.

In one example, a blend of at least two diisocyanates is reacted with areactive diol to produce the polyurethane backbone. The reactive diolmay be a methacrylate-containing and/or acrylate-containing reactivediol. In some examples, the reactive diol may be anacrylamide-containing reactive diol, an allyl-containing reactive dioland/or a styrene-containing reactive diol. In one example, the reactivediol may be a methacrylate-containing or acrylate-containing diol thatis bisphenol A-free. As shown above, examples of such diols include:

The polyurethane backbone may be devoid of any ionic side groups, forexample, acid stabilisation groups (e.g. carboxylic or sulphonic acidgroups). Such ionic groups may act as capping groups at the terminalend(s) of at least some of the polyurethane polymer strands in thepolyurethane dispersion.

In one example, the polyurethane dispersion is formed by reacting areactive diol with a diisocyanate to form a polyurethane pre-polymer. Apolymerisation initiator may be used to initiate polymerisation. TheNCO/OH ratio may range from greater to 1 to 8, for example, 1.2 to 5.

The polymerisation may be carried out to produce a polyurethanepre-polymer. Once the polyurethane pre-polymer is formed, a cappingagent may be added to the reaction mixture. For example, the cappingagent may be a monoalcohol or monoamine e.g. selected from amethacrylate-containing monoalcohol or monoamine, an acrylate-containingmonoalcohol or monoamine, an acrylamide-containing monoalcohol ormonoamine, a styrene-containing monoalcohol or monoamine, anallyl-containing monoalcohol or an allyl-containing monoamine. Themonoalcohol or monoamine may react with terminal —N═C═O groups on thepolyurethane pre-polymer to cap the polyurethane pre-polymer. Thereaction is carried out such that at least some of the polyurethanepre-polymer strands are capped by this reaction. In some examples, mostof the polyurethane pre-polymer strands are capped by this reaction. Forexample, at least 10% of unreacted —N═C═O groups are capped by thisreaction. In some instances, 50 to 95%, for instance, 60 to 90% ofunreacted —N═C═O groups are capped by this reaction.

An amino carboxylic acid or an amino sulphonic acid may then be added tothe reaction mixture. As mentioned above, suitable acids includetaurine, 3-(cyclohexylamino)-1-propanesulfonic acid and2-(cyclohexylamino)ethanesulfonic acid. The amino carboxylic acid oramino sulphonic acid may react with the remaining —N═C═O groups. Thesegroups form can form ionic capping groups that help to stabilise thedispersion of polyurethane in e.g. water.

The pH stable curable polyurethane may have an acid number of 20 to 100.The pH stable curable polyurethane may have a double bond density from1.5 to 1.0 meq/g.

The particle size range of the pH stable polyurethane dispersion may be10 to 200 nm.

Any suitable surfactant may be present in the inkjet ink composition.Suitable surfactants are described in relation to the overcoatcomposition above. The surfactant employed in the inkjet ink compositionmay be the same or different from the surfactant used in the overcoatcomposition. When present, the surfactant present in the inkjet inkcomposition in an amount ranging from about 0.01 wt % to about 5 wt %based on the total wt % of the inkjet ink composition.

The inkjet ink composition may include a co-solvent in addition towater. Classes of co-solvents that may be used can include organicco-solvents, including alcohols (e.g., aliphatic alcohols, aromaticalcohols, polyhydric alcohols (e.g., diols), polyhydric alcoholderivatives, long chain alcohols, etc.), glycol ethers, polyglycolethers, a nitrogen-containing solvent (e.g., pyrrolidinones,caprolactams, formamides, acetamides, etc.), and a sulfur-containingsolvent. Examples of such compounds include primary aliphatic alcohols,secondary aliphatic alcohols, 1,2-alcohols, 1,3-alcohols, 1,5-alcohols,ethylene glycol alkyl ethers, propylene glycol alkyl ethers, higherhomologs (C6-C12) of polyethylene glycol alkyl ethers, N-alkylcaprolactams, unsubstituted caprolactams, both substituted andunsubstituted formamides, both substituted and unsubstituted acetamides,and the like. Still other examples of suitable co-solvents includepropylene carbonate and ethylene carbonate.

A single co-solvent may be used, or several co-solvents may be used incombination. When included, the co-solvent(s) is/are present in total inan amount ranging from 0 wt % to 60 wt %, depending on the jettingarchitecture, though amounts outside of this range can also be used. Asother example, the co-solvent(s) may range from about 1 wt % to about 30wt % or about 20 wt % of the total wt % of the inkjet ink composition.

The inkjet ink composition may also include various other additives toenhance the properties of the ink composition for specific applications.Examples of these additives include those added to inhibit the growth ofmicroorganisms, viscosity modifiers, materials for pH adjustment,sequestering agents, anti-kogation agents, preservatives, and the like.Such additives may be present in an amount of 0 to 5 wt % of the inkjetink composition.

Printing Process

As described above, the present disclosure relates to a process thatcomprises inkjet printing an inkjet ink composition onto a substrate. Aradiation-curable (e.g. analogue) overcoat composition may be appliedover the printed inkjet ink composition on the substrate. Thereafter,the printed inkjet ink composition on the substrate is cured by exposingboth the overcoat composition and inkjet ink composition on thesubstrate to radiation.

The overcoat composition may be applied using any suitable technique.For example, the overcoat may be applied by brush, roller, knife,spraying or inkjet ink printing. The overcoat may be applied to form aovercoat layer that is 0.1 to 5 microns thick, for example, 0.5 to 4microns thick or 1 to 3 microns thick.

Prior to the inkjet printing the inkjet ink composition onto thesubstrate, the substrate may be treated, for example, with coronatreatment. Prior to inkjet printing the inkjet ink composition onto thesubstrate, a fixer composition may be applied to the substrate. Thefixer may help to optimise the deposition of pigment onto the printsubstrate. Suitable fixers may include one or more calcium salts andwater. Examples of suitable calcium salts include calcium nitratetetrahydrate and calcium propionate. In one example, the fixer comprisescalcium nitrate tetrahydrate and calcium propionate. Surfactant may alsobe present in the fixer composition. A suitable surfactant may be anon-ionic surfactant, for example, Surfonyl® SEF. The fixer may alsoinclude a biocide. A suitable biocide may be Acticide® B20. Water may bepresent in the fixer in an amount of 5 to 30 weight % of the fixercomposition. The fixer may be applied after the substrate has beentreated by corona treatment. In some examples, the fixer may be appliedto a substrate that has not been corona treated.

Any suitable inkjet ink printing method may be used. Examples includethermal and piezoelectric inkjet printing. In some examples, thermalinkjet printing is employed.

After the inkjet ink composition is printed onto the substrate, it maybe dried prior to application of the overcoat composition.

Once the overcoat is applied, it may be dried prior to the curing step.

Any suitable source of radiation may be used to cure the inkjet ink andovercoat. In one example, UV radiation is employed. Suitable sources ofUV radiation include UV lamps, LED (light emitting diode) lamps, LEP(light emitting plasma) plasma torches, or lasers operating in the UVrange.

The actual wavelength (within the UV range of 280 nm to 400 nm) andintensity of the ultraviolet radiation used may vary, depending at leastin part, upon the curable polyurethane in the binder and inkjet inkcomposition. Examples of suitable UV LED wavelengths include 365 nm, 385nm, 395 nm or 405 nm, for example, 365 nm and 395 nm.

The printing process of the present disclosure can be used to print on abroad selection of substrates, including untreated plastics, flexible aswell as rigid, porous or non-porous substrates. Some examples includepaper (e.g., plain paper, coated, glossy paper, etc.), cardboard, foamboard, textile, and plastics. Examples of suitable plastic substratesinclude vinyl substrates, for example, vinyl graphic films availablefrom 3M™ under the trademark Scotchcal™ series IJ-40. Other examplesinclude acrylic substrates, for example, acrylic cast graphic filmsavailable from 3M™ under the trademark Controltac™ (e.g. 180-10(cast)).Other examples include acrylic glass substrates (PMMA), polypropylenesubstrates, polystyrene substrates (e.g. high impact polystyrenesubstrates), PVC substrates and polycarbonate substrates.

FIG. 1 depicts schematically, by way of example only, an example of thesequence of steps that may be taken to perform an example inkjetprinting process of the present disclosure. As can be seen from theFigure, a fixer 10 may be applied to a print substrate 12. An ink jetink composition 14 may then be inkjet printed over the fixer. Theprinted ink layer may be dried by a dryer 16. Thereafter, an overcoatcomposition 18 may be applied (e.g. by an analogue method e.g. using aroller) over the printed ink layer as an overcoat layer. The overcoatlayer may then by dried 20. Thereafter, the over coated print layer maybe cured by exposure to UV-LED 22.

To further illustrate the present disclosure, examples are describedbelow. It is to be understood that these examples are provided forillustrative purposes and are not to be construed as limiting the scopeof the present disclosure.

EXAMPLES Example 1—Synthesis of Curable pH Stable Acrylamide-Based PUD

38.884 g of g of BGDA (see compound XVII above), 0.389 g of4-methoxyphenol (MEHQ), 42.103 g of 4,4′-Methylene dicyclohexyldiisocyanate (H12MDI), and 42 g of acetone were mixed in a 500 ml of4-neck round bottom flask. The reaction mixture was stirred and theflask immersed in a constant temperature bath at 60° C. 3 drops of DBTDLwas added to initiate the polymerization. Polymerization was continuedfor 3 hrs at 60° C. 0.5 g samples was withdrawn for % NCO titration toconfirm the reaction. The measured NCO value was 7.6%. Theoretical % NCOshould have been 8.32%. 12.318 g of HEAA (N-hydroxylethyl acrylamide,CAS #7646-67-5, purchased from Sigma Aldrich), 0.159 g of MEHQ, and 19 gof acetone were mixed in a beaker and added to the reactor over 30 sec.9 g of acetone was used to rinse off the residual monomers on the beakerand added to the reactor. The polymerization was continued 3 hours at50° C. 0.5 g of pre-polymer was withdrawn for final % NCO titration. Themeasured NCO value was 2.41%. The theoretical % NCO should have been2.41%. The polymerization temperature was reduced to 40° C. 6.695 g oftaurine, 4.494 g of 50% NaOH, and 33.474 g of deionized water are mixedin a beaker until taurine is completely dissolved. Taurine solution wasadded to the pre-polymer solution at 40° C. with vigorous stirring over1-3 mins. The solution became viscous and slight hazy. Continue to stirfor 30 mins at 40° C. The mixture became clear and viscous after 15-20mins at 40° C. Add cold 194.649 g of deionized water to polymer mixturein 4-neck round bottom flask over 1-3 mins with good agitation to formpolyurethane dispersion (PUD). The agitation was continued for 60 minsat 40° C. The PUD dispersion was filtered through 400 mesh stainlesssieve. Acetone was removed with rotorvap at 50° C. (add 2 drops (20 mg)BYK®-011 de-foaming agent if there is a lot of foaming). The final PUDdispersion was filtered through fiber glass filter paper. Particle sizemeasured by Malvern Zetasizer is 26.8 nm. Its pH was 6.0. Solid contentwas 30.04%. This PUD shows less than 0.4 unit pH drop after 1 weekaccelerated shelf life testing at elevated (60 degrees C.) temperature(ASL).

Example 2

The curable pH stable PUD produced in Example 1 was used to formulate aninkjet ink formulation.

Component Weight % Surfactant (Surfonyl ® CT-211, supplied by 0.80AirProducts ®) Dynax ® DX-4000 (fluorsurfactant, supplied 0.50 byDynax ®) Water soluble photoinitiator (TPA Na) 0.50 Water solublesensitizer² 0.25 curable PUD of Example 1 3-11.00¹ Black pigment 2.5Water Balance ¹0.9 to 3.3 weight % polyurethane (solids) dispersed inink composition; ²A photosensitizer based on a functionalised anthronemoiety coupled to a polyether chain, see Q above.

Example 3

A curable overcoat composition was prepared having the followingcomposition: 35% polyurethane dispersion (Ucecoat® 7571 from Allnex®),3% surfactant (Tegowet® 510) and 0.3% photo initiator (Irgacure®,commercially available from BASF Corp. 819DW).

Example 4

In this example, a fixer composition was applied to a vinyl substrate(Avery Dennison® IC MPI 2900). Thereafter, the inkjet ink compositionsof Example 2 were inkjet printed over the substrate using a thermalinkjet printer. The printed inkjet ink composition was dried and thencoated with the overcoat composition of Example 3. The printed substratewas then dried and cured using UV-LED at 395 nm.

The cured substrates were subjected to tests to determine theirdurability with respect to a Windex rub test (1 weight (250 g), 5cycles, Crockmeter cloth), a 70% IPA rub test (1 weight (250 g), 5cycles, Crockmeter Cloth), and an eraser rub test (1 weight (250 g), 10cycles). The tested substrates were inspected by visual inspection. Therub tests (Windex, 70% IPA. Eraser rub) were graded with a score of 0(best) to 5 (worst). The results are shown in table 1 below.

TABLE 1 wt % of curable PUD of Example 1 in ink tested/(wt %polyurethane solids in ink 75 70% Eraser tested) OD L* Gloss Windex IPArub 5 (1.5) 1.49 23 90 0 0 2 7 (2.1) 1.53 21 91 0 0 1 9 (2.7) 1.59 20 930 0 1 11 (3.3) 1.61 19 89 0 0 1

The tested substrates were also found to have excellent optical density(OD), L* (Lightness) and gloss.

Example 5

The procedure of Example 4 was repeated with a coated paper substrate,Graph+. The cured substrates were tested according to the Windex rubtest and 70% IPA rub test as described in Example 4 above. In addition,the cured substrates were tested by a smear rub test (4 in. length, 1700g weight, Mellotex), a heated Sutherland rub test (177 degrees C. (350degrees F.), 27.6 kPa (4 psi) heated weight, speed 2, 10 cycles) and aquanta regular Sutherland rub test (4 lbs, 200 cycles, Mellotex). Forthe Sutherland tests, a score of 5 is indicative of excellent rubresistance, while a score of 1 shows poor rub resistance. The resultsare shown in Table 2 below. Smear rub mean values of around 238 wereconsidered to be good.

TABLE 2 wt % of curable PUD of Example 1 in ink tested/(wt %polyurethane Quanta solids in ink 75 70% Smear Heated Regular tested) ODL* Gloss Windex IPA Rub Sutherland Sutherland 3 (0.9) 1.99 10 94 0 0239.4 5 4 5 (1.5) 2.10 6 94 0 0 238.1 5 4 7 (2.1) 2.08 6 94 0 0 238.8 54 9 (2.7) 2.16 7 92 0 0 238.5 5 5 11 (3.3) 2.07 8 93 0 2 238.5 4 3

Examples 4 and 5 above show that, by using an overcoat composition,durable images can be produced. In contrast, in the absence of theovercoat compositions, curing of the black ink compositions was found tobe very difficult. This is believed to be because of UV absorption bythe black colorant, which reduced the efficiency of the curing process.Furthermore, the images produced in the absence of the overcoat had poordurability and very poor scores were achieved, particularly with respectto IPA rub resistance.

1. An inkjet printing process comprising inkjet printing an inkjet inkcomposition onto a substrate to form a printed inkjet ink layer, theinkjet ink composition comprising a colorant, a curable polyurethanedispersion, a photoinitiator and water, wherein the amount of curablepolyurethane dispersed in the inkjet ink composition is 0.1 to 30 weight%, applying a radiation-curable overcoat composition over the printedinkjet ink layer as an overcoat layer, said overcoat compositioncomprising a curable polyurethane dispersion, a photoinitiator andwater, and curing the printed inkjet ink layer on the substrate byexposing both the overcoat layer and printed inkjet ink layer on thesubstrate to radiation.
 2. A process as claimed in claim 1, wherein thepolyurethane in the overcoat layer and the polyurethane in the printedinkjet layer are crosslinked during curing to form a crosslinkedpolyurethane network.
 3. A process as claimed in claim 1, wherein theovercoat layer and printed inkjet ink layer on the substrate are curedby exposure to UV-LED.
 4. A process as claimed in claim 1, wherein thecurable polyurethane is present in an amount of 0.5 to 5 weight % of theinkjet ink composition.
 5. A process as claimed in claim 1, wherein thecurable polyurethane is present in an amount of 1 to 30 weight % of theovercoat composition.
 6. A process as claimed in claim 1, wherein thecurable polyurethane dispersion in the inkjet ink composition is a pHstable curable polyurethane dispersion.
 7. A process as claimed in claim6, wherein the pH stable curable polyurethane dispersion comprises apolyurethane polymer comprising a polyurethane backbone having at leastone terminal group selected from at least one of anacrylamide-containing group, a styrene-containing group, anallyl-containing group,


8. A process as claimed in claim 6, wherein the pH stable curablepolyurethane dispersion comprises a polyurethane polymer comprising apolyurethane backbone that is capped at one end with a terminal groupselected from at least one of an acrylamide-containing group, astyrene-containing group and an allyl-containing group and at theopposite end with a terminal group selected from carboxylic acid-,carboxylate anion-, sulphonate anion- and sulphonic acid-containinggroup.
 9. A process as claimed in claim 6, wherein the pH stable curablepolyurethane dispersion comprises polyurethane polymer comprising abackbone that is capped at one end with a terminal group selected froman acrylamide-containing group, a styrene-containing group, anallyl-containing group, a methacrylate-containing or acrylate-containinggroup, and at the opposite end with a terminal group selected from atleast one of:


10. A process as claimed in claim 6, wherein pH stable curablepolyurethane dispersion comprises a polyurethane polymer formed from thereaction of a) a blend of at least 2 isocyanates and b) a reactive diol.11. A process as claimed in claim 1, wherein the curable polyurethane inthe inkjet ink composition is different to the curable polyurethane inthe overcoat composition.
 12. A process as claimed in claim 1, whichfurther comprises drying the printed inkjet ink layer on the substrateprior to application of the overcoat composition.
 13. A process asclaimed in claim 1, which further comprises drying the overcoat layer onthe substrate prior to curing.
 14. A process as claimed in claim 1,wherein the concentration of polyurethane in the overcoat composition isgreater than the concentration of polyurethane in the inkjet inkcomposition.
 15. A printed substrate comprising an ink layer comprisinga colorant disposed over the substrate and an overcoat layer disposedover the ink layer, wherein the printed substrate comprises acrosslinked polyurethane network that surrounds the colorant and extendsfrom the ink layer to the overcoat layer.